1. Field of Endeavor
The present invention relates to a novel construction for diaphragms of the type used in axial flow turbomachines. It is particularly, but not exclusively, relevant to steam turbine diaphragms.
2. Brief Description of the Related Art
The present invention is related to the so-called “platform” type of diaphragm construction, see FIGS. 1A and 1B. FIG. 1A is a perspective view of a static blade or vane, and FIG. 1B is a view on a radial section of a diaphragm during manufacture, including the static blade. In this type of diaphragm, the ends of the aerofoils 1 are integral with radially inner and outer “platforms” 2, 3, the aerofoils and platforms being machined from solid. In FIG. 1A, an adjacent blade shape is shown in dashed lines, a complete annulus of static blades being built up by assembling successive combined aerofoil/platform components 4 into an annular array between inner and outer diaphragm rings 5, 6 and welding the platforms to the diaphragm rings. The inner and outer diaphragm rings and platforms are further machined as appropriate to accommodate turbine sealing features and to fit adjacent turbine features. When the assembly is finished, the inner and outer platforms 2, 3 form the inner and outer port walls of the diaphragm.
The current practice for HP and IP steam turbines employing platform construction is to build the blades onto the inner diaphragm ring and then to shrink the outer diaphragm ring on to the blades. In current designs, the inner diaphragm ring is required to support the static blades and to give the diaphragm rigidity against forces that tend to distort it during assembly and operation of the turbine.